The present invention relates to communication over a subscriber line. More specifically, it relates to a method for optimizing downstream data transfer in a splitterless asymmetric digital subscriber line modem.
Digital Subscriber Line (xe2x80x9cDSLxe2x80x9d) is a developing modem technology that allows existing copper telephone lines to carry high bandwidth information. Familiar twisted-pair telephone lines are then able to carry high speed data communication to and from a customer site in addition to retaining a plain old telephone service (xe2x80x9cPOTSxe2x80x9d) channel for voice communication. At the customer site, a DSL modem receives the downstream signal representing data for a customer from a central office for a telephone company and transmits an upstream signal representing data from the customer to the central office.
One configuration of DSL is generally termed Asymmetric Digital Subscriber Line (xe2x80x9cADSLxe2x80x9d). ADSL allows high-speed data transport to the customer site but only medium-speed data transfer from the customer site, whence the asymmetry. ADSL also allows the simultaneous use of the ADSL modem and POTS on the same telephone line. In this configuration, at a point where the twisted-pair telephone line enters the customer site, filters split the POTS channel from the ADSL modem channels. One problem with this type of ADSL, however, is that a technician from the telephone company has to visit the customer site and install the splitter, referred to as xe2x80x9cthe truck roll.xe2x80x9d
To overcome this problem, another configuration of ADSL forgoes the requirement of a splitter at the customer site. This configuration is generally called xe2x80x9csplitterless ADSLxe2x80x9d and also goes under the name of xe2x80x9cG.Lite.xe2x80x9d Splitterless ADSL is described in the International Telecommunications Union-Telecommunication Standardization Sector (hereinafter xe2x80x9cITU-Txe2x80x9d, formerly known as the CCITT) Recommendation G.992.2, February 1999, which is incorporated herein by reference. ITU-T standards can be found on the World Wide Web at the Universal Resource Locator (xe2x80x9cURLxe2x80x9d) xe2x80x9cwww.itu.ch.xe2x80x9d Splitterless ADSL modems include those manufactured by 3Com Corporation of Santa Clara, Calif., Lucent Technologies of Murray Hill, N.J., Texas Instruments of Dallas, Tex., and others.
In splitterless ADSL, an ADSL modem directly shares the same twisted-pair telephone line as a POTS telephone without the intervention of a splitter. Sharing the same twisted-pair telephone line, however, may introduce noise into the downstream data transfer. The noise effect is the response of the internal circuitry of the telephone to the upstream ADSL signal. Moreover, each telephone has a different response to the upstream ADSL signal: some telephones introduce little downstream noise, such as many of the cordless telephones, while other telephones introduce a lot of noise, thus severely reducing the rate at which the modem can receive data. It is desirable, therefore, to reduce the downstream noise introduced by the telephone to optimize the downstream data transfer. It is also desirable to achieve noise reduction for a wide range of telephone models. This may allow customers to use their telephones while the ADSL modem is transferring data.
In accordance with preferred embodiments of the present invention, some of the problems associated with overcoming downstream noise in a splitterless ADSL modem are overcome. The modem implements a power cutback to enable simultaneous use of a telephone and the modem, where the telephone and modem share a telephone loop.
One aspect of the invention includes a method of optimizing downstream data transfer from a central office to the modem. The method includes ascertaining a first noise characteristic for the telephone loop when the telephone is on-hook, and measuring a second noise characteristic for the telephone loop when the telephone is off-hook. The modem determines whether the first noise characteristic is less than the second noise characteristic, and if so, the modem limits its transmitted power by an amount equal to a power cutback value. In this manner, the method may enable simultaneous use of the telephone and the modem, while optimizing downstream data transfer.
The foregoing and other features and advantages of preferred embodiments of the present invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.